1. Field of the Invention
The present invention relates to an electrolyte composition used in a dye-sensitized solar cell, a dye-sensitized solar cell and a production method thereof.
2. Related Background Art
A photoelectric conversion element using a semiconductor such as silicon converts a ray of light such as sunlight and a laser beam into electric energy, and a solar cell utilizing the photoelectric conversion element receives attention as a petroleum-replacement energy source and as an energy source that emits no CO2. There are used many types of photoelectric conversion elements in the solar cell, such as a monocrystalline silicon type, an amorphous silicon type, a polycrystalline silicon type and other compound types, but these have the following problems. Specifically, the photoelectric conversion element of the monocrystalline silicon type is produced in the same process as in the semiconductor production, which needs an expensive cost, though the element provides a high conversion efficiency. Further, because of its low coefficient of light absorption, it is required to have a thickness of some extent (preferably 50 μm or more), which increases the quantity of expensive high-purity silicon to be used, and consequently increases the material cost. On the other hand, the polycrystalline silicon type may be produced at a lower production cost, but cannot reduce the thickness of the expensive silicon material for the solar cell, and still needs a high material cost. Under these circumstances, there have been research works for developing an amorphous silicon solar cell that can be produced to have a large area at a low cost. However, this solar cell still employs the same raw material, and it is hard to say that it has an adequate cost-cutting effect.
On the other hand, a certain device attracts attention as one form of a photoelectric conversion device, which differs from a conventional solar cell, does not employ Si as a raw material, but as described in Japanese Patent Application Laid-Open No. H01-220380, comprises a first transparent electrode, a transparent semiconductor provided thereon, a colorant adsorbed on a surface of the transparent semiconductor and having a sensitizing effect, a charge transportation layer provided thereon, and a second transparent electrode on the charge transportation layer. Such a device does not need an expensive raw material of high-purity Si, but uses inexpensive an oxide semiconductor such as titanium oxide and zinc oxide, to significantly reduce the raw material cost. Further, the device also does not need a production apparatus for use under high vacuum in contrast to that for a Si solar cell, may greatly reduce the cost for a production facility and may become a great step for the spread of solar cells.
The photoelectric conversion device operates in the process described below.
When incident light reaches a colorant having a sensitizing effect through a first transparent electrode and a transparent semiconductor, or the sensitizing colorant through a second transparent electrode and a carrier layer, the light excites the colorant having the sensitizing effect to generate electrons in a LUMO level and holes in a HOMO level. The electrons produced by excitation in the LUMO level of the sensitizing colorant immediately move to the conduction band of the transparent semiconductor and migrate to the first transparent electrode. The holes remaining in the HOMO level of the sensitizing colorant receive electrons from a carrier-transfer layer, whereby the sensitizing colorant is neutralized. Ions or holes produced in the carrier-transfer layer by the impartment of electrons diffuse in the carrier-transfer layer, reach the second clear electrode, and receive electrons from the second transparent electrode. By using the first transparent electrode that has received the electrons as a negative electrode, and the second transparent electrode that have imparted the electrons as a positive electrode, it is possible to obtain an electrical signal or an electric power converted from the incident light pattern.
However, such a photoelectric conversion element employs a liquid electrolyte, which poses the problem of durability thereof.
Main factors responsible for deterioration seen in long-duration use include (1) leak or vaporization of an electrolyte, (2) short circuiting between electrodes, (3) deterioration of dye, (4) peeling off of a porous electrode, and (5) deterioration of a transparent electrode. Of these factors, (1) the leak or vaporization of the electrolyte is the largest issue, and it is very difficult in a production process to seal the liquid on a large area so as to endure a long period of light irradiation.
Recently, research works on an electrolyte using a solvent with a high boiling point such as polyethylene glycol, and an electrolyte using an ionic liquid as a main component, which is a liquid at ordinary temperature and does not vaporize, have been energetically conducted. The electrolytes developed in these research works scarcely vaporize, and such devices as to employ the electrolytes and have consequent superior durability are published. However, as long as the employed electrolyte is liquid, a large amount of an electrolyte liquid can leak from even a fine fracture in a sealed part through capillary phenomenon, so that it is very difficult to maintain a completely sealed state for a long period of time in a system using a flexible substrate, and an encapsulating material has to be prudently selected and the encapsulating step has to be perfect in a significantly high level.
For the purpose of avoiding the difficulty of encapsulating, a research work for using a solid electrolyte has been conducted. There have been conducted a research work for using copper iodide of a P-type semiconductor, and a research work for using an organic hole-transfer layer of an electroconductive (hereinafter, simply referred to as conductive) polymer. However, these solid electrolytes cannot maintain the characteristics for a long period of time or have low conversion efficiency, and sufficient solid electrolytes have not been yet obtained.
The electrolyte regarded as the most hopeful for solving these problems is now a gelation of a liquid electrolyte. There proposed are the research works for using a low-molecular type gelating agent published in Chem. commun. 2002, 374; a method of introducing a reaction precursor into a cell, effecting polymerization to constitute a matrix, and then effecting impregnation with an electrolyte liquid thereto published in chem. lett. 948, 2002; and a method of constituting a network of a micro-layer-separation structure in an electrolyte liquid by using a vinylpyridine polymer and a polyfunctional halide as crosslinking agent to conduct gelation, published in chem, lett. 918 (2002).
In any of the methods, the obtained performance of the gelated electrolytes are equal to or nearly equal to that of a liquid electrolyte, but in order to produce the gelated electrolyte, it is necessary to inject a reaction liquid into an encapsulated cell and promote the gelation by heat or the like, so that the production process is more complicated than that of using a normal liquid electrolyte.
In Japanese Patent Application Laid-Open No. 2003-226766, a method is studied in which a porous film is formed of an HFP-PVDF copolymer and impregnated with an electrolyte liquid to provide a film-shaped electrolyte. The method is considered to be very useful as one of methods for simplifying the handling of an electrolyte. This photoelectric conversion device uses a plastic film having a transparent conductive film of ITO formed thereon as a medium can be continuously produced in a roll-to-roll system, so that the cost is considered to be significantly reduced. However, in order to impart a predetermined pattern to an electrolyte, a step of disposing a formed film-shaped electrolyte at a predetermined location, or a step of imparting the film to the whole area and appropriately removing an unnecessary part is considered to be necessary, and a step, an apparatus and an alignment method therefor are considered to be necessary.